Wide band dipole antenna with capacitive reactance added to arms

ABSTRACT

A wide-band antenna comprising a dipole antenna mainly consisting of a pair of linearly arranged antenna arms and a capacitor disposed at a prescribed one point on each of said paired arms equally spaced from the center of a feeding portion.

United States Patent [191 Hidaka [451 July 17,1973

WIDE-BAND DIPOLE ANTENNA WITH 'CAPACITIVE REACTANCE ADDED TO ARMS lnventor: Kazutaka Hidaka, Yamato-shi,

Kanagawa-ken', Japan Assignee: Tokyo Shlbaura Electric Co., Ltd.,

Kawasaki-shi, Japan Filed: May 24, 1971 Appl. No.: 146,412

Foreign Application Priority Data May 25, 1970 Japan 45/43989 US. Cl 343/749, 343/802, 343/819 Int. Cl. H011 9/00 Field of Search 343/722, 744, 802,

[56] References Cited UNITED STATES PATENTS 2,666,138 1/1954 Ehrbar et al 343/749 2,667,577 1/1954 Graziano 343/815 2,715,184 8/1955 Cork 343/749 2,976,532 3/1961 Guest 343/722 Primary Examiner-Eli Lieberman Attorney-Kemon, Palmer and Estabrook 57 ABSTRACT A wide-band antenna comprising a dipole antenna mainly consisting of a pair of linearly arranged antenna arms and a capacitor disposed at a prescribed one point on each of said paired arms equally spaced from the center of a feeding portion.

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Kan-mum mama Kim", Pal-mar L ESTABQcoL WIDE-BAND DIPOLE ANTENNA WITH CAPACITIVE REACTANCE ADDED TO ARMS This invention relates to a wide frequency band antenna and more particularly to a wide frequency band antenna having a capacitor disposed at those points on an antenna element which is equally spaced from the center of the feeding portion of said antenna element. At present there are used a wide variety of antennas for reception of broadcast television signals. The Yagi antenna in particular is widely accepted in receiving especially VHF and UHF television signals. However, the Yagi antenna is generally only capable of receiving as narrow frequency bands as lass than 150 MHz, failing to cover the entire range of 470 to 770 MHz bands of UHF television signals used, for example, in Japan. Therefore, there have been made attempts to provide an antenna which can receive such wide frequency bands, and research work has been concentrated on improvement of a radiator constituting the main part of antenna, which has heretofore obstructed reception of wide frequency bands. As a result, there have been developed, for example, a dual fan type antenna, broad diameter dipoleantenna and folded antenna. Though improved to receive a little wider frequency bands, said proposed antennas are not yet sufficient to cover the entire range of 470 to 770MHz frequency bands used in the UHF television broadcasting, still less suitable for reception of all 470 to 890 MHz frequency bands for said broadcasting adopted by foreign countries. The aforementioned antennas have to be formed of expen sive metal plates, particularly, the large diameter dipole antenna requiring considerable material cost. Further, these antennas are of complicated construction, consuming a great deal of time and work in fabrication. Therefore, there has been demanded the developement of a wide frequency band antenna which is of simple construction and displays wide frequency band properties in order to eliminate the aforesaid drawbacks, and

to this end, there have been made further approaches. For illustration, T. S. M. Maclean states in the Proceed ings of I.E.E., Vol. 115, No. 10, October, 1968 that if there is provided a capacitor at a plularity of, for example, fouror six points on a dipole antenna-element which'are equally spaced from the center of its feeding portion, then the antenna can receive much broader frequency bands. Maclean experimentally concludes in said publication that sinceit is practically inefi'ective simply todispose a capacitor at an arbitrary single point on eachof a pair of antenna arms constituting an antenna element, there should be provided a capacitor at several points on each antenna arm which are respectively arranged in symmetrical relationship with respect to the central point of the antenna element. What should be noted here is that Maclean made theoretical calculations in connection with the case where there was used a capacitor only at one point on each antenna arm, but did not eventually discover the conditions required for an antenna to display fully desirable properties. His further experiments based on the aforesaid calculations show that formation of a single capacitor on each antenna arm did not enable the resulting antenna to receive wide frequency bands. However, he did not eventually succeed in distinctly defining the conditions for an antenna to exhibit wide frequency band properties. The aforesaid linear symmetrical arrangement of a plurality of capacitors on both antenna arms with respect to the center of an antenna element as proposed by Maclean inevitably leads to the reduced mechanical strength of said arms and difficulties in the manufacture of an antenna device.

It is accordingly an object of this invention to provide a wide frequency band antenna having a capacitor provided at two points on an antenna element equally spaced from the center of its feeding portion, that is, one capacitor on each of a pair of antenna arms constituting the antenna element in linear, as well as electrically serial arrangement, thereby receiving wide frequency bands with the voltage standing wave ratio (hereinafter referred to as VSWR) chosen to be less than 2.5, a practically permissible range including matching and mismatching.

Another object of this invention is to provide a wide frequency band antenna constructed by disposing a capacitor of, for example, 0.5 to 5 PF at two points on the radiator of a Yagi antenna equally spaced from the center of its feeding portion in linear, as well as electrically serial arrangement, thereby receiving all frequency bands of UHF television signals adopted all over the world.

The present invention can be more fully understood from the following detailed description when taken in conjunction with reference to the appended drawings, in which:

FIG. 1 is a perspective view of a wide frequency band antenna according to an embodiment of this invention;

FIG. 2 is a substantially equivalent representation of the wide frequency band antenna of FIG. 1;

FIG. 3 to 5 are concrete sectional views of that portion of an antenna element where there is formed the capacitor of FIG. 1;

FIG. 6 is another concrete sectional view of the capacitor portion of FIG. 1;

FIG. 7 shows the arrangement of antenna arms as viewed from the top of FIG. 6;

FIGS. 8 to 10 are Smith charts showing the properties of a wide frequency band antenna according to an embodiment of the invention;

FIG. 11 is a perspective view of a Yagi antenna to the radiator of which there is applied this invention;

FIG. 12 is a plan view of another antenna arm using the invention; and

FIG. 13 is a curvediagram showing the properties of the Yagi antenna of FIG. 11.

There will now be described by reference to the appended drawings the construction of a wide frequency band antenna according to this invention. FIG. I presents a dipole antenna according to the invention. To the opposite sides of an electrically insulated antenna arm holder.3 provided with feeder terminals 1 and 2 are fitted a pair of antenna arms 4 and 5 to constitute an antenna element or dipole antenna. The feeder terminals I and 2 are connected to a feeder (not shown). The inner ends of the antenna arms 4 and 5 are electrically connected to the feeder terminals 1 and 2 in the antenna arm holder 3. There are formed two capacitors 6 and 7 in linear, as well as electrically serial arrangement, one on each arm at a prescribed point equally spaced in the opposite directions from the middle point of the feeder terminals] and 2, that is, the central point 0 of the antenna element. The arrangement of the dipole antenna of FIG. 1 is equivalently presented in FIG. 2, in which the capacitors 6 and 7 are both designated as C The distance between the capacitors C is indicated by 21,, the total length of the antenna by 21 the radius of that portion of the antenna arms 4 and 5 which extends from the central point of the feeding portion to each capaciter C, by p, (in FIG. 2 the diame ter of said portion is taken and indicated by 2p,), and the radius of that portion of the antenna arms 4 and 5 which extends from the outer end thereof to each capacitor C, by p (in FIG. 2 the diameter of said portion is taken and indicated by 2p The present inventor has found the conditions required for an antenna to display desired wide frequency band properties simply by disposing, as mentioned by reference to FIGS. 1 and 2, a capacitor on each of a pair of antenna arms constituting a dipole antenna at such an equal distance as meeting the later described conditions from the center of the feeding portion of said dipole antenna.

There will now be described by reference to FIGS. 3 to 6 the various concrete constructions of the capaciters C,. Since the paired antenna arms 4 and 5 are of the same construction, description is given of only one arm 4. Referring to FIG. 3, the arm 4 formed of a hollow conductor is divided substantially at the center into two sections 8 and 9. The facing ends of these divisions 8 and 9 are separated at a prescribed space. There is disposed coaxially with the arm 4 a central conductor 10 of prescribed length which intersects at right angles a plane including the end face of each division and extends well thereinto. The interior of the hollow arm divisions 8 and 9 including a space between the facing ends thereof is packed with a dielectric material 11 such as phenol resin or acrylonitrile-butadiene-styrene terpolymer in a manner to surround the central conductor 10 and also fill up the space between the facing ends of the arm divisions 8 and 9. On the outer peripheral surface of that part of the antenna arm 44 which is filled with said dielectric material 11 is deposited another insulating material 12 of the same kind as or a different kind from said interior dielectric material 11 so as to cause both arm divisions 8 and 9 to be integrally fixed in place. This construction enables a capacitor C, to be formed between the hollow antenna arm 4 and the central conductor 10 as well as between the facing ends of the arm divisions 8 and 9. These capacitors have a tatal capacitance equal to C, shown in FIG. 2.

Referring to FIG. 4, there is coaxially inserted into a hollow conductor 13 a rod conductor 14 having a smaller diameter than the former for a prescribed length jointly to constitute an antenna arm 4 or 5. The space defined by the inserted portion of the rod conductor 14 with the inner walls of the hollow conductor 13 is filled with a dielectric material 11. On the outer peripheral surface of that part of the hollow conductor 13 where the rod conductor 14 is coupled therewith is deposited an insulating material 12 so as to integrally fix both conductors l3 and 14. This construction causes a capacitor to be formed between both conductors 13 and 14 with a capacitance corresponding to the volume of a space defined therebetween plus the prescribed areas of the facing surfaces thereof. The aforesaid rod conductor 14 may be substituted by a hollow conductor having a prescribed smaller diameter than the outer hollow'conductor 14.

Referring to FIG. 5, there are provided columnar or plate conductors 15 and 16 with the prescribed surfaces thereof disposed to face each other and a specified gap allowed therebetween in order to form a capacitor C, having a desired capacitance. The interspace between both conductors l5 and 16 is filled with a dielectric material 11. On the outer peripheral surface of those portions of said conductors l5 and 16 which overlap each other, as well as of the fractional extensions of said portions, there is deposited another insulatint material 12. In this case, the capacitor C, is formed mainly between the interfaces of both conductors 15 and 16.

The capacitor C, may be formed otherwise as shown in FIGS. 6 and 7. Referring to FIG. 6, conductors 17 and 18 formed of, for example, hollow aluminum tubes to constitute antenna arm 4 or 5 have the facing ends compressed flat and inclined in opposite directions with respect to the axis of said conductors 17 and'l8 to constitute flat end portions 19 and 20. The conductors l7 and 18 are so arranged as to cause said inclined flat end portions 19 and 20 to face each other at a prescribed space. In the interspace between the inclined flat end portions 19 and 20 and on the preipheral surface of said end portions 19 and 20 as well as of the fractional extensions thereof there is disposed a dielectric material of, for example, phenol resin or acrylonitrile-butadiene-styrene terpolymer to cause both conductors 17 and 18 to be integrally fixed, and also a capacitor C, to be formed essentially between said facing flat end portions 19 and 20. Said conductors l7 and 18 may consist of hollow cylindrical types having different diameters. The aforementioned construction enables the flat end portions 19 and 20 to act as a stopper with respect to the dielectric material 11 thereby preventing the conductors l7 and 18 from being displaced in the axial directionor rotating around the axis, when subjected to an external force.

While the object of this invention is attained by forming a capacitor at a prescribed point on each of the paired antenna arms equally spaced from the center of the feeding portion ofa dipole antenna in linear, as well as electrically serial, relationship, the present inventor has discovered proper interrelationships among the various conditions required for an antenna to display practically sufficient wide frequency band properties.

There will now be described those interrelationships among the various conditions required to obtain said wide frequency band properties which the invention has distinctly defined.

The inventor has theoretically calculated input admittance Yin as viewed from the feeding portion of a dipole antenna shown in FIGS. 1 and 2 and indicated said admittance by the following equation:

where:

Za input impedance as viewed from the center 0 of the feeding portion when there is applied no load, namely, when the capacitor C, is short-circuited Zb impedance as viewed from the position of that of the two capacitors C, disposed at an equal space I, from the center 0 of the feeding portion which is removed upon short-circuiting said center 0, while the other capacitor is retained Zm mutual impedance between the center 0 of the feeding portion and a point displaced for a distance I, from the center 0 when the two capacitors are removed 0) angular frequency For better understanding of the following description, the equation (1) above has been converted as follow by substituting input impedance Zin for the aforesaid input admittance Yin.

Zin: 1

A wave length l,,= natural logarithm The factors l,, 1 p, and p denote the measurements of the various parts of the antenna shown in FIG. 2. When the properties of said antenna are to be studied there may be admitted all possible combinations of .variables. However, description of such combinations would prominently increase the number of variables to be considered, presenting difficulties in broadly envisaging the properties of the antenna. Accordingly, reference is made, only as often as need arises, to the variables used in the tests and studies conducted to accomplish this invention. To proceed with further description of the invention, there is made an assumption represented by the equation (6) obtained from said tests and studies.

z/Pz). MPz/Pr) From the equation (6) above, 0., may be taken to be equal to 0,. The reason is that there results from the equation (5) the following equation:

Therefore, from the equations (6) there results (l, 2ln(2l,/p Q

Where a used as a proportion constant is assumed to be 1, then I, may be expressed as follows:

I, al

There will be given further description by initially choosing the proportion constant a to be 0.5 so as to meet the case where there is provided a capacitor C, at the center of the antenna arms (4)and(5).

The term jmC, of the equations (1) and (2) above may be expressed as follows:

j l j 2'( r/ z) where:

c light speed (3 X 10 cm/sec) Thus the present inventor has found that the input impedance Zin of the equation (2) may be indicated as functions of the variables of a, K1,, 0, and C,/l,. FIGS. 8, 9 and 10 are Smith charts in which the input impedance Zin is indicated in combinations of these variables. It will be noted that said Smith charts used in the following description represent a normalized impedance of 300 0. FIGS. 8, 9 and 10 indicate Zin varying in proportion to the value of Kl, with C,/l a and I), taken as variables. The inventor has also found that as seen from said charts, Zin can be defined within the range of VSWR of less than 2.5 by setting the variables of C,/l a and Q, at a proper value. His experimental results also satisfactorily proved this fact.

FIG. 8 represents the case where the normalized impedance was set at 300 0., a at 0.5, and (I, at 8.65, showing Zin curves obtained by choosing the variable C,/l, to be 0.0417, 0.0625 and 0.0834. As described above, I), is a particularly important factor in defining the ratio which the radius of the antenna arm bears to its length. In FIG. 8, 2.2 to 4.0 denote the values of K1,, the capacitance of a capacitor C, included in the variable C,/l is indicated in pico-farads, and l, in centimeters.

These units are also used in FIGS. 9 and 10. The relationship of KI, and frequency is presented in Table below.

TABLE f(MHz) Kl, l,,=20 cm l =22 cm l =24 cm 1.8 430 392 360 1.97 470 429 390 2.0 I 478 435 400 2.16 469 470 432 2.2 526 478 440 2.35 561 511 470 2.4 574 52] 480 2.6 621 565 520 2.8 669 610 560 3.0 716 653 600 3.2 765 696 640 3.22 770 700 641 3.4 812 740 680 3.54 845 770 v 708 3.6 859 783 720 3.8 907 826 760 3.85 918 836 770 4.0 955 890 800 The K1 value of 2.16 to 3.54 in Table above obtained by choosing 1 to be 22 cm corresponds to a wide frequency band of 470 to 770 MHz. Referring to the Zin curve of FIG. 8 representing C,/I, 0.0834, the value of Kl, can be so defined as to cause VSWR to fall within the fully satisfactory range of less than 2.5. In this case the capacitance of the capacitor C, may be calculated as 0.0834 X22 1.8 PF. FIG. 8 further shows that as C,/l has increasing values, the associated curve is successively shifted to the adjacent outer curves, namely,

resulting in larger VSWR values. Said increase in the value of C,/l means a corresponding decline in the capacitor C, formed on the antenna arm with respect to its fixed length 1 As apparent from the foregoing description, when there is disposed a capacitor of small capacitance at a prescribed point on each of the paired antenna arms, equally spaced from the center of the feeding portion of a dipole antenna, the desired frequency band can be defined within the practically permissible range of the VSWR value, thus enabling an antenna to receive much broader frequency bands than has been possible with the conventional antenna. I-Iowever,'indiscriminate choice of the capacitance of the capacitor C fails to allow an antenna to display practically useful wide frequency band properties. Tests, studies and calculations conducted on a dipole antenna using this invention show that unless Kl representing the ratio of the frequency to the length of the antenna arm is reduced to below 4.1, the desired object can not be attained, and unless'C,/l denoting the ratio of the capacitance of the capacitor C to the length of the antenna arm is defined within the range of 0.025 to 0.3 PF/cm for practical application, there will not be obtained a prominent effect of enabling an antenna to receive wide frequency bands.

The foregoing description by reference to FIG. 8 relates to the case where the proportion constant a indicating the required position of the capacitor C that is, 1 /1 was chosen to be 0.5. However, FIG. 9 presents the curves of the impedance Zin when the normalized impedance was set at 300 .Q, Q, at 8.65, C,/I at 0.0625 and the proportion constant a at 0.416, 0.5 and 0.583 respectively. This figure shows that as the proportion constant has increasing values, the associated curve is successively shifted to adjacent outer curves, and conversely where a has decreasing values, Zin is represented by progressively inner curves. Therefore, it will be seen that though the value of C ll may be fixed, proper choice of the impedance Zin can be easily effected by changing the proportion constant a, namely, the position of the capacitor C FIG. 10 indicates the curve of the impedance Zin when the normalized impedance was set at 300 Q, C,/l at 0.0625, a at 0.5 and Q, at 7.75, 8.65 and 9.2 respectively. In this case, as Q, has increasing values, the impedance Zin is represented by progressively outer curves. Tests show that if .0 was chosen to be 8.65 with respect to an antenna desired to receive wide frequency bands of 470 to 770 MHz, then such antenna would well serve practical purpose. It has been found that 0, should generally be choosen to be less than 12. The term (I, of the equation (6') is also taken to be less than 12.

As mentioned above, if the conditions of Kl 4.l, 0.025 C,/I 0.3 PF/cm and Q l2 are fully met, there will be obtained a dipole antenna capable of receiving wide frequency bands which can be defined within the range of VSWR of less than 2.5. If the radiator 20 of the Yagi antenna of FIG. 11 consists of this dipole antenna, said Yagi antenna will be able to receive wide frequency bands. The aforesaid dipole antenna may be formed by arranging a plurality of antenna arms 21 and 22 in parallel with each other in a horizontal plane or by bending the facing ends of said arms as indicated in broken lines for connection. The modification of FIG. 12 is equivalent to a single dipole antenna whose thickness has been considerably increased.

The foregoing description may be summarized as follows:

A. The ratio of the capacitor C to the distance 1 from the outer end of the antenna arm to the center 0 of the feeding portion, that is C /l is defined to be 0.025 to 0.3 PF/cm.

B. A value 0, twice the natural logarithm of the ratio of the total length 2!; of the dipole antenna to the radius p, of that part of the antenna arm which extends from the center 0 of the feeding portion of the antenna element to the capicitor, that is, 2ln(2l /p,), and a value 0 twice the natural logarithm of the ratio of the entire length 2l of the antenna element to the radius p, of that part of the antenna arm which extends from the position of the capacitor to the outer end of said arm, that is 2ln(2l /p are both chosen to be less than 12.

C. The ratio of a distance I, from the center line of said feeding portion to the position of the capacitor which bears to a distance I, from the center 0 of the feeding portion of the antenna element to the outer end of the antenna arm, that is, l ll a is set at 0.4 to 0.6.

If a dipole antenna is so constructed as to meet the above-mentioned conditions, then it will be possible to receive much wider frequency bands than has been possible with the prior art antenna by defining them within the range of a prominently favorable VSWR value of, for example, 2.5. The reduced diameter of an antenna element decreases material cost, making it possible to provide an inexpensive wide frequency band antenna. Application of this invention particularly to the Yagi antenna enables it to display excellent wide frequency band properties. FIG. 13 illustrates the properties of the Yagi antenna of eight elements, which is constituted of director, reflector and radiator elements 213, 24 and 20, to the radiator element 20 of which there is applied this invention. The curve diagram of FIG. 13 represents the case where a distance l from the center of the feeding portion of the Yagi antenna radiator of FIG. 11 to the outer end of the antenna arm was set at 20 cm, the proportion constant a at 0.5, the diameters 2p and 2p of the different portions of said arm both at 1.27 cm and the capacitor C, at 5 PF. As apparent from FIG. 13, all frequency bands of 470 to 790 MHz used in UHF television broadcasting can be received by being defined within the range of voltage standing wave ratio of less than 2.5. When, therefore, the Yagi antenna to which there is applied this invention is used in receiving television signals, then it will improve the picture quality and sensitivity of a TV receiving set, thus effectively serving practical application.

What is claimed is:

l. A wide band dipole antenna comprising:

a feeding portion;

a pair of arms extending in line with each other in opposite directions from said feeding portion; and

a pair of capacitors, one disposed in each of said arms respectively, located at points equally spaced from the center of said feeding portion, and connected electrically in series with said arms;

in which C ll is between 0.25 and 0.3;

2ln(2I /p and 2ln(2l /p are each less than l,/I is between 0.4 and 0.6

where C, is the capacitance of said capacitor in picofarads I, is the distance between said capacitor and the center of said feeding portion 1 is the distance from the end of either arm to the center of said feeding portion 12; and,

p is the radius of said arms between said feeding portion and said capacitor p is the radius of said arms between said capacitor and the outer ends of the arms In is the base of natural logarithms all physical measurements being in centimeters.

2. A wide-band antenna according to claim 1 wherein said capacitor is formed of those end portions of pair of hollow conductors substantially constituting the antenna arm which face each other at a predetermined space; a central conductor of predetermined length coaxially disposed with the hollow conductors in the interspace between said facing end portions; an inner dielectric material filled in an area defined by the inner walls of the facing end portions of the hollow conductors with the outer walls of the central conductor as well as in an interspace between said facing end portions; and an outer insulation material mounted on the outer peripheral surface of said facing end portions, as well as of the fractional extensions of said end portions.

3. A wide-band antenna according to claim 1 where said capacitor is formed of the end portion of a first hollow conductor substantially constituting the antenna arm; that end portion of a second conductor having a smaller diameter than said first hollow conductor and inserted coaxially therewith at a space for predetermined distance which faces the end portion of said first hollow conductor; an inner dielectric material filled in said first hollow conductor so as to surround the aforesaid end of the second conductor; and an outer insulation material deposited on the outer peripheral surface of the end portion of the first hollow conductor as well as of the fractional extension of the end portion of the second conductor.

4. A wide-band antenna according to claim 1 wherein said capacitor consists of those portions of a pair of plate conductors substantially constituting the antenna arm which face each other at predetermined space; an inner dielectric material filled between said facing end portions; an outer insulation material deposited on the outer peripheral surface of those portions of the paired conductors between which there is filled said dielectric material as well as of the fractional extensions of said portions.

5. A wide-band antenna according to claim 1 wherein said capacitor is constituted by flattened end portions 'of a pair of hollow conductors substantially forming the antenna arms which are inclined in opposite directions with respect to the axis of said conductors so as to face each other at a predetermined space; and a dielectric material deposited between said facing flattened end portions and on the outer peripheral surface of said end portions as well as of the fractional extensions thereof.

6. A wide band yagi antenna comprising director. elements, a reflector element and a dipole element as defined by claim 1. 

1. A wide band dipole antenna comprising: a feeding portion; a pair of arms extending in line with each other in opposite directions from said feeding portion; and a pair of capacitors, one disposed in each of said arms respectively, located at points equally spaced from the center of said feeding portion, and connected electrically in series with said arms; in which C1/l2 is between 0.25 and 0.3; 2ln(2l2/ Rho 2) and 2ln(2l2/ Rho 1) are each less than 12; and, l1/l2 is between 0.4 and 0.6 where C1 is the capacitance of said capacitor in picofarads l1 is the distance between said capacitor and the center of said feeding portion l2 is the distance from the end of either arm to the center of said feeding portion Rho 1 is the radius of said arms between said feeding portion and said capacitor Rho 2 is the radius of said arms between said capacitor and the outer ends of the arms ln is the base of natural logarithms all physical measurements being in centimeters.
 2. A wide-band antenna according to claim 1 wherein said capacitor is formed of those end portions of pair of hollow conductors substantially constituting the antenna arm which face each other at a predetermined space; a central conductor of predetermined length coaxially disposed with the hollow conductors in the interspace between said facing end portions; an inner dielectric material filled in an area defined by the inner walls of the facing end portions of the hollow conductors with the outer walls of the central conductor as well as in an interspace between said facing end portions; and an outer insulation material mounted on the outer peripheral surface of said facing end portions, as well as of the fractional extensions of said end portions.
 3. A wide-band antenna according to claim 1 where said capacitor is formed of the end portion of a first hollow conductor substantially constituting the antenna arm; that end portion of a second conductor having a smaller diameter than said first hollow conductor and inserted coaxially therewith at a space for predetermined distance which faces the end portion of said first hollow conductor; an inner dielectric material filled in said first hollow conductor so as to surround the aforesaid end of the second conductor; and an outer insulation material deposited on the outer peripheral surface of the end portion of the first hollow conductor as well as of the fractional extension of the end portion of the second conductor.
 4. A wide-band antenna according to claim 1 wherein said capacitor consists of those portions of a pair of plate conductors substantially constituting the antenna arm which face each other at predetermined space; an inner dielectric material filled between said facing end portions; an outer insulation material deposited on the outer peripheral surface of those portions of the paired conductors between which there is filled said dielectric material as well as of the fractional extensions of said portions.
 5. A wide-band antenna according to claim 1 wherein said capacitor is constituted by flattened end portions of a pair of hollow conductors substantially forming the antenna arms which are inclined in opposite directions with respect to the axis of said conductors so as to face each other at a predetermined space; and a dieLectric material deposited between said facing flattened end portions and on the outer peripheral surface of said end portions as well as of the fractional extensions thereof.
 6. A wide band yagi antenna comprising director elements, a reflector element and a dipole element as defined by claim
 1. 